Most current bioanalytical assays are inaccessible for developing economies. Current diagnostic assays typically require large and expensive laboratory instruments that are operated by trained personnel. Thus, there remains a need for low-cost diagnostic assays that are not cumbersome and that can be performed on small sample volumes. Further, there remains a need for low-cost systems to detect trace levels of analytes in fluids for, e.g., (i) human health; (ii) illicit drug use; (iii) military and homeland security settings; and (iv) chemical pollution in the environment.